Recently, an ultrasound system has been extensively used in the medical field due to its non-invasive and non-destructive nature. Modern high-performance ultrasound imaging diagnostic systems and techniques are commonly used to produce two- or three-dimensional ultrasound images of internal features of patients. In order to provide the ultrasound images, the ultrasound system operates in various image modes such as a brightness mode, a Doppler mode and the like to acquire ultrasound images for diagnosis.
In the Doppler mode, the ultrasound system provides a color flow image showing velocities of moving objects such as blood flow, heart, etc. The color flow image may be formed based on a Doppler signal obtained by alternately transmitting and receiving ultrasound signals to and from a target object. The Doppler signal may include a low frequency signal (the so-called clutter signal) due to the motion of a cardiac wall or valve of a heart. The clutter signal may have amplitude, which is over 100 times than that of a pure Doppler signal indicative of velocities of the blood flow. The clutter signal may be an obstacle to accurately detect a velocity of the blood flow. Thus, it is required to remove the clutter signal from the Doppler signal for accurate velocity detection of the blood flow. The ultrasound system typically adopts a clutter filter, which may be a high pass filter, to remove the clutter signal from the Doppler signal.
Recently, two clutter filtering methods have been adopted to remove the clutter signal from the Doppler signal. One is a clutter filtering method using a clutter power threshold. The other is a clutter filtering method using variance information of the clutter signal.
The clutter filtering method using the clutter power threshold may be carried out by modulating the Doppler signal when the amplitude of the Doppler signal is greater than the clutter power threshold. Thereafter, the modulated Doppler signal may be filtered by using a clutter filter having a predetermined cutoff frequency. In such a method, however, if the amplitude of the Doppler signal is less than the clutter power threshold, then the modulation may not be performed. Also, when the power level of the pure Doppler signal is close to that of the clutter signal, the modulation may not be desirably performed. Thus, the clutter signal may not be effectively removed from the Doppler signal.
The clutter filtering method using the variance information of clutter signal may be carried out by changing a cutoff frequency of a clutter filter. The clutter filter adopted in the conventional ultrasound system may remove the clutter signal in the Doppler signal by suppressing the frequency components thereof below a specific cutoff frequency. In such a case, however, if the cutoff frequency is set to a high value, then relatively low frequencies of the pure Doppler signal may be also cut off along with the clutter signal. Thus, it may be difficult to accurately detect the velocities of the blood flow.